The present invention relates to the production of structures associating a superconducting material with a mechanical reinforcement material, possessing good thermal characteristics. An example of such a structure is that of sheets, or narrow tubes, of niobium associated with a rigidification layer, for example in copper or in tungsten.
Such structures offer applications in the field of particle accelerators.
FIG. 1A represents an accelerating structure of an electron accelerator. Such a structure takes the form of successive cavity cells 2-1, . . . , 2-9. The particles are accelerated here by a radio-frequency wave generated by a klystron. Length L is 1039 mm for a frequency of 1.3 GHz.
FIG. 1B represents a structure for accelerating protons. Cells 2-10, . . . , 2-13 in niobium are immersed in a bath 3 of liquid helium. Such a structure has a diameter D of 1.1 m for a frequency of 700 MHz.
The shape and the dimensions of these cavities are optimised according to a large number of parameters linked to the RF performance, dark currents, turbulence field etc. Resolution of the Maxwell equations associated with the conditions at the limits on the walls makes it possible to define space and time values for the electric and magnetic fields in such a structure.
These electric and magnetic fields contribute to the accelerating effects on the particles of the beam, but also to secondary effects, in particular the heating of materials and structures, dark currents etc.
In particular, a current j induced in the cavity walls leads to a loss of high frequency power.
At present, there is a distinction between two types of accelerators: the so-called xe2x80x9chotxe2x80x9d accelerators made with copper cavities, and the xe2x80x9csuperconductorxe2x80x9d accelerators using cavities of a superconducting material such as niobium, which is cooled below its critical temperature TC to make it superconducting. The critical temperature TC for niobium is 9.3 K, which implies cooling the structure in a bath of liquid helium (at atmospheric pressure helium is liquid at 4 K).
In the first case, for hot accelerators, a large part of the electrical power provided by the network serves to heat the cooling water in the copper structures. In the case of a superconductor accelerator, the greater part of the electrical power serves to accelerate the particle beam, which helps explain all the interest of superconductivity in terms of electrical consumption. However, a small part (but not zero) of the HF energy is dissipated in a fine layer of superconducting material called the LONDON layer. The typical thickness of the LONDON layer is about 100 nm and does not depend on the frequency, as is the case for currents induced in a normal conductor. This dissipated energy makes it possible to explain the variations in the characteristic curve of a superconductor cavity Q(Eacc), or quality factor, depending on the accelerating field. Since the dissipated energy rises with the accelerating field, the characteristic curve follows a descending slope depending on the field.
The BCS theory on superconductivity worked out by Bardeen, Cooper and Schrieffer in 1957 makes it possible to predict the corresponding resistance, (the so-called BCS resistance) and the losses from the Joule effect, which depend on the frequency and temperature.
To this resistance RSCS, one should add a residual resistance linked to the defects of structures, interstitial atom impurities, included gases etc.
Thus, in a structure of superconducting cavities, one can consider that the thickness of the superconducting material (niobium, for example), constituting the superconducting cavity, plays several roles:
1xe2x80x94The role of superconducting layer as an internal skin, on the vacuum side of the cavity.
2xe2x80x94The role of heat sink for the rest of the thickness, allowing the calories generated by the BCS and residual resistances in the LONDON layer to flow towards the helium bath.
3xe2x80x94The role of mechanical structure making it possible to conserve the internal shape fixing the conditions at the limits to the electromagnetic field which is created in the structure of the tube and in the accelerating cavities.
The optimisation mentioned above makes it possible to reach compromises allowing in general the optimisation of the RF (or HF) shapes and specifications but leaves open the questions of mechanical stability and the thermal properties of such structures.
When an accelerating machine is used, the mechanical and geometric conditions must remain stable in order to maintain the structure of the cavities tuned to the klystron frequency. Various causes may disturb this operating stability: Lorentz forces creating pressure in the cells and tending to deform them, mechanical vibrations induced from outside, and in particular induced by variations in the pressure of the liquid helium bath etc.
In transient states, when the HF wave is introduced into the cavities, the structure is submitted to Lorentz forces which tend to deform and detune it. To avoid this, rigid structures need to be produced.
An example of such a rigidified structure is illustrated in FIG. 2A. A corrugated tube 4 is produced with a high thickness of superconducting material, generally niobium, which is expensive. In fact, the tube is an assembly of elementary parts 5, 7, 9, 11 assembled by welds 6, 8, 10.
The utilisation of thick niobium means using a large quantity of very expensive material (between 1,500 FF and 5,000 FF per kilo), a procedure which can scarcely be envisaged for machines operating with a high number of cavities.
Another known structure, represented schematically in FIG. 2B, consists of making cavity parts in material 12 of average thickness, of welding them together with welding seams 16 and strengthening them with a ring 14 at the level of the iris (regions or zones of lower diameter). Consequently, in order to overcome the effect of the Lorentz forces, the utilisation of niobium of lower thickness necessitates welding stiffeners 14, complicating the production process and making it difficult to control the dimensions because of shrinking after welding. In addition, such a technique makes it difficult to obtain reproducible dimensions.
Another technique (FIG. 2C) consists of depositing a thin layer 22 of niobium on a substrate 20 in thick copper. This process makes it possible to solve the problem of rigidifying the structure. However, the structure obtained has limits in terms of the accelerator field which can be reached. In fact, for reasons linked to the structure of the superconducting layer of niobium deposited by xe2x80x9csputteringxe2x80x9d on the copper substrate, the maximum electric fields likely to be reached remain of the order of 10 MV/m. For other machines, and in particular colliders e+-exe2x88x92, this field is clearly insufficient.
Apart from the mechanical problems, there are also thermal problems. Thermal conditions must be maintained so that the internal skin of the niobium remains below the critical temperature TC and below the critical field HC. In order that the thermal conditions are maintained and the niobium remains superconducting in the LONDON thickness, it is necessary that if there is a hot spot on the internal surface, on the vacuum side of the accelerator, the calories can be evacuated rapidly towards the helium bath.
There are several reasons which can lead to the creation of a hot spot:
1)xe2x80x94HF losses through the Joule effect, due to the uniformly distributed global and homogeneous surface resistance described by the BCS theory and by the residual resistance. A more complete theory introducing non-quadratic losses, presented by W. Weingarten (xe2x80x9cProgress in thin film techniquesxe2x80x9d, CERN-European Laboratory for Particle Physics, Geneva, Switzerland, 7th Workshop on RF Superconductivity, Paris, 1995), shows that it is possible to express super-losses by the following formula, giving the superconducting resistance (measured in nxcexa9/mT):                                                                                           R                  s                                ⁡                                  (                                                            B                      ρ                                        ,                    ω                    ,                    T                    ,                                          B                      ext                                                        )                                            =                              xe2x80x83                            ⁢                                                                                                                  R                        0                                            ⁡                                              (                        ω                        )                                                              T                                    ⁢                                      exp                    ⁡                                          [                                              Δ                                                  KT                          c                                                                    ]                                                                      +                                                      R                    res                                    ⁡                                      (                                          ω                      ,                                              B                        ext                                                              )                                                  +                                                                                                        xe2x80x83                            ⁢                                                                                          R                      s                                        ⁡                                          (                                              ω                        ,                        T                        ,                                                  B                          ext                                                                    )                                                        ·                                      B                    ρ                                                  +                ⋯                                                                        (        1        )            
The first term gives the BCS losses, the second the losses due to residual resistance and the third the non-quadratic losses.
The losses corresponding to the second and third terms are explained by non-superconducting metallic inclusions such as tantalum which, after the metallurgical processes for working niobium, are still present in the niobium matrix, and also by dissolved impurities (oxygen, carbon etc.)
2)xe2x80x94The HF losses due to the field emission and, possibly, also due to electron emissions through the thermoionic effect. According to the Fowler-Nordheim theory, above a certain surface field, electrons are extracted from the surface. They can then be accelerated by the electromagnetic fields present in the structure.
These electrons, emitted by the field effect and accelerated by the electromagnetic wave, can then collide with the structure of the cavities, in another place, and dissipate their kinetic energy in the form of heat.
3)xe2x80x94Dielectric HF losses, from dust and contaminants of a dielectric nature which may have been left inside the cavities during the manufacturing process.
To minimise these xe2x80x9chot spotxe2x80x9d sources, extremely pure niobium must thus be used, work must be carried out at low temperatures, smooth surface conditions must be obtained, and all dust deposits on the HF side must be avoided.
In addition, if one wishes to work with a high accelerating field, and thus as a consequence with a high magnetic field as well in the region of the equator, it is not possible to avoid creating heat. As Padamsee showed in xe2x80x9ccalculations for breakdown induced by large defects in supraconductive niobium activitiesxe2x80x9d, published in IEEE Translations on Magnetics, vol. 19, 1983, even in the absence of defects, if one reaches the critical field HC locally, the material passes from the superconducting state to the normal state.
Below this limit, the heat thus produced at the internal wall of the cavity must be evacuated, as efficiently as possible, towards the helium bath, in such a way as to limit the rise in temperature of the internal wall of the cavity.
If the speed of access to the cold source is insufficient, a localised thermal perturbation may spread and lead eventually to a breakdown of the cavity, called xe2x80x9cquenchxe2x80x9d in anglo-saxon jargon (transition of the HF wall from the superconducting state to the normal resistive state).
Thus a xe2x80x9cquenchxe2x80x9d or xe2x80x9cthermal breakdownxe2x80x9d generally originates in a region where a higher resistance exists, or in a xe2x80x9cnon conductionxe2x80x9d zone, or a defect or a foreign particle on the surface of the superconducting material.
If, locally, at a defect, more heat is produced than can flow in the direction of the helium bath, the temperature of the zone rises and tends to become a zone of xe2x80x9cnon-superxe2x80x9d conductivity, which then extends until the whole of the energy stored in the cavity is dissipated in the hot region.
Thus one understands that one tries to obtain high thermal conductivity of the cavity wall to avoid this problem of xe2x80x9cthermal breakdownxe2x80x9d, and also low values of accelerating fields.
Moreover, the power density generated by the currents in the LONDON layer is proportional to the square of the local magnetic field BS. Yet, the value of the local magnetic field B(s), along the meridian, has a maximum value obtained at the equator (the zone with the biggest diameter).
From the equator, and in the direction of the iris, BS reduces slowly, then more rapidly when one passes from the equator to the iris.
A defect will therefore not be as harmful situated at the equator as it will be situated very far from the equator, towards the iris. In addition, the equator zone is especially sensitive from the point of view of superconductivity defects since the Foucault currents induced at the level of normal electrons, in the inside skin, will be greater in this zone because of the high value of the magnetic field.
Consequently, the xe2x80x9charmfulnessxe2x80x9d of a defect is not identical, depending on its geographic location in the cavity. Moreover, all things being equal, situated near the equator, in a zone where the magnetic field is at a maximum, it will have a tendency to show more harmfulness vis-à-vis the xe2x80x9cquenchxe2x80x9d than if it is situated in the neighbourhood of the iris.
On the other hand, a defect on the surface or very close to the surface, in the neighbourhood of the iris, will be more sensitive to the electric field and will be likely to emit electrons according to the Fowler-Nordheim law, or possibly even (according to the Richardson law), simply by thermoionic effect.
When one produces superconducting cavities following the present process, using niobium sheets which are pressed and welded from the outside by electron beam, defects are introduced. Impurities are localised at the fusion bath and therefore, for the process used at present, at the lower end of the weld seam, on the vacuum side of the structure.
Since, in addition, the present process consists of making a weld at the equator and that one has seen above that this is the zone with the highest magnetic field, this process is very likely to create a hot spot in this region.
Moreover, since niobium has an affinity for oxygen, there needs to be a very good vacuum in the vessel when the welds are carried out by electron beam. Studies have shown that, during the welding process, the bath of metal in fusion absorbed oxygen from the vessel and thus created locally a zone where the purity of the niobium was debased.
All these considerations demonstrate the difficulties which have to be overcome at the industrial level to produce, in a reliable manner and with reproducible results, an object which has as many xe2x80x9cbadly placedxe2x80x9d welds (from the point of view of xe2x80x9csuperxe2x80x9d defects) as those shown in FIGS. 2A or 2B. Moreover, it is difficult to obtain reproducible vacuum conditions at levels as low as 10xe2x88x929 torr when welding by electron beam.
The present structures, of the type shown in FIGS. 2A and 2B, use a large number of welds and these zones are especially critical, above all those located at the equators.
In addition, the welding techniques practised have a tendency to concentrate the impurities towards the inside (where the LONDON layer is located) and this, among other things, raises the residual resistance.
In order to combat the xe2x80x9cquenchxe2x80x9d phenomenon, or the thermal runaway of the cavity, following the appearance of a hot spot, known structures need to use materials of great purity with a high thermal conductivity (corresponding to an RRR at least higher than 200), that is to say materials whose purification costs come on top of the usual costs of standard industrial materials. (The RRR, the xe2x80x9cResidual Resistance Ratioxe2x80x9d, is a measure of the purity of the material, involving defects in structure and microscopic or macroscopic defects. It is also defined by the relationship between the cold electrical resistivity and the resistivity at ambient temperature).
But, in the case of niobium, even of one uses very pure niobium, one should note that the thermal conductivity of this material is not very good compared, for example, with that of copper or aluminium. In addition, in the case where the heat flux created by a hot spot cannot be absorbed by the thickness of niobium, one should note that the niobium/liquid helium interface resistance is not negligible.
Thus, the structures and processes described above do not make it possible to obtain both the mechanical stability and the thermal conditions required, in particular for high accelerating fields.
Documents JP-0 2220399 and JP-0 2220400 (Patent Abstracts of Japan, OEB) suggest a special production technique for superconducting niobium cavities whose walls are covered with a metal which is a good thermal conductor using a process of application by plasma spraying.
Thanks to this technique one can produce a superconducting cavity, comprising a sheet or tube in superconducting material, with the appropriate shape and with improved thermal properties.
However there remains the problem of rigidity of the cells and the cavity.
As mentioned above, the rigidity of a cavity with a multiple cell structure can be raised by welding stiffeners in the shape of rings (FIG. 2B).
Apart from the above-mentioned problems of mechanical restrictions due to shrinkage of the welds, it seems that the presence of rings is scarcely compatible with the technique of plasma spraying.
An aim of the present invention is to propose an accelerator cavity and a manufacturing process for such a cavity making it possible to solve the problems laid out above.
A particular aim is to propose such an accelerator cavity offering excellent thermal properties and mechanical rigidity at an especially low cost.
The rigidity of the cavity can certainly be increased by applying a thicker plasma-projected layer of metal. However, the production of a thick layer of metal, apart from the cost it represents, becomes long and complex, taking into account the conditions for plasma spraying.
Thus, in order to achieve the above aims, the invention has more precisely the objective of a particle accelerator cavity with a string of multiple cells, the cells presenting a region with a higher diameter, called the equator region, and end regions of lower diameter called iris which link the cells between them, the cells being delimited by a wall in a material with superconducting properties, which is covered by at least one layer of thermal conducting material, characterised in that the layer of thermal conducting material has a thickness which is greater in the iris regions than in the equator region of the cells.
It is to be noted that by simply increasing the thickness of the thermal conducting material in the iris regions of the cells, it is possible to raise significantly the mechanical rigidity of the cavity. The non-uniform character of the thickness in fact makes it possible to combat efficiently the Lorentz forces acting on the wall. Thus, the setting of rings or other rigidity reinforcements becomes superfluous.
The lower thickness of the thermal conducting material in the equator regions does not prejudice the rigidity.
A smaller quantity of thermal conducting material can be used and the application time for this material can be reduced. The manufacturing costs of the cavity are thus lowered.
In addition, when higher rigidity of the cavity is obtained, the thickness of the superconducting material can be reduced as well. This also contributes to lowering the costs.
A cavity conforming to the invention can be used particularly for electron or proton accelerators.
As mentioned above, the thermal conducting material can be applied by plasma spraying.
Plasma spraying makes it possible to obtain a porous structure producing an interface whose total developed surface can be greater than that obtained by prior art. This increase in the exchange surface makes it possible to improve the thermal exchanges between the liquid helium and the possible heat source which could develop locally.
The increase in the exchange surface between materials makes it possible to reduce the Kapitza resistance, or interface thermal resistance, which is one of the physical properties determining the thermal performance of the superconducting structure.
The coating process by plasma spraying, depending on the size of the constitutive particles of the powders, and according to the settings of the plasma torch, makes it possible to obtain porous layers whose porosity can be adjusted.
Besides the thermal advantage explained already, such a layer, while rigidifying the structure, also makes it possible to absorb efficiently the vibrations of the corrugated skin of the superconducting material.
In the case where the superconducting material is niobium, one can increase the exchange surface further by advantageously spraying a thin layer of niobium on the external face of the structure, before spraying the copper or the second material to rigidify the structure. In addition, in order to facilitate thermal exchanges, the layer of thermal conducting material can be coated with a layer of a material with acoustic impedance lower than that of the thermal conducting material.
In fact, one can improve the Kapitza resistance between the copper (or the plasma deposited material) and the liquid helium by spraying, for example, a layer of aluminium making the acoustic adaptation between two elements, one solid and with a high acoustic impedance and the other liquid with low acoustic impedance.
In the case where the shape of the cells allows it (that is, in the case where the relationship between the diameter at the equator and the diameter of the iris is not too high), one can shape the cavity from a tube without welding, deformed by a known procedure such as hydroforming, tube expanding, hot working, hydrosparking, magnetic forming etc. Once the tube has been obtained with its corrugations, instead of welding an external ring, as in prior art (FIG. 2B), the structure is rigidified by spraying powder externally, for example copper, on the outside surface. One can also use tungsten or any other material possessing good thermal properties.
In the case where the dimensions of the cavities are such that one cannot use a process of tube shaping without welding, without tearing the metal, one can start from thin pressed sheet elements joined by welds carried out by laser beam or electron beam according to known techniques. Since a weld at the equator presents a significant risk, one can advantageously offset it to another location.
Once the structure with its corrugations has been obtained, comprising successive cells, the ensemble is rigidified externally by plasma spraying of the thermal conducting material as described above.
By thermal conducting material one means a material possessing good thermal properties enabling the xe2x80x9cquenchxe2x80x9d to be evacuated. For example, copper and tungsten are good candidates.
A further aim of the invention is a process for making an accelerator cavity comprising a plurality of cells with equator regions of higher diameter and iris regions of lower diameter, and delimited by a wall of a material with superconducting properties, in which a layer of thermal conducting material is formed on the surface of the said wall by plasma spraying. According to the invention, the thermal conducting material is sprayed in such a way that it forms a thicker layer in the iris regions than in the equator regions.